Natural Enemies - Pathogens

Research Issue

Anoplophora glabripennis (ALB) (Coleoptera: Cerambycidae), a large and destructive wood-boring beetle native to China and Korea, has been accidentally introduced to North America and Europe in solid-wood packing materials used to ship manufactured goods. ALB attacks and eventually kills hardwood species in at least six tree genera: birch (Betula), elm (Ulmus), horse chestnut (Aesculus), maple (Acer), poplar (Populus), and willow (Salix). The ecological and economic risks are high, and in the U.S. alone, ALB could result in loss of 35% of our forest canopy or 1.2 billion trees valued at $669 billion; losses to lumber, maple syrup, nursery, and tourist industries may exceed $41 billion. For this reason, regulatory agencies are attempting to eradicate ALB through the detection, removal, and destruction of infested trees, and the injection of nearby healthy host trees with systemic insecticides. Should eradication fail, however, methods for managing ALB will be needed. A regional, integrated pest management approach will be required, including the use of biological and microbial controls, host plant resistance, silviculture, and systemic insecticides.

Our Research

We studied natural enemies attacking ALB in the U.S. and China. The goals were to 1) identify natural enemies impacting ALBin the U.S. and 2) provide data on factors affecting ALB population dynamics in its native range. Should management tools be needed in the future, this information may prove useful for development of a biological control program for ALB. In the U.S., when trees were being felled during the eradication program from 1999 to 2002, we sampled ALB from infested Norway maples and box elders in New York City and from Norway and silver maple, box elder, and American elm in Chicago. In China, we sampled live and dead ALB from infested poplar, willow, and/or elm in Gansu, Hebei, Henan, Hubei, Jiangxi provinces, and Inner Mongolia Autonomous Region of China. Most of the field sites were sampled once during the season, however, the field site in Gansu province was sampled in July and October 2000, June 2001, and April 2002. Most ALB were sampled as larvae although pupae, adults, and eggs were also found.
Depending on disease signs and symptoms, ALB cadavers were either necropsied or cultured upon return to the laboratory. The natural enemies were identified to species when possible. Pathogenic fungi were cultured and deposited in the USDA ARS Collection of Entomopathogenic Fungal Cultures in Ithaca, NY. Microsporidia were cultured and studied in ALB and the cottonwood borer (Plectrodera scalator), a native cerambycid, analyzed using transmission electron microscopy and genetics, and spores purified and stored in liquid nitrogen in our laboratory for future research. Insect parasitoids and predators were stored in ethanol.

Expected Outcomes

If eradication of ALB is unsuccessful in North America, we anticipate the need for biological control, which is a long-term management strategy used for the sustained control of invasive species. Biological control typically requires foreign exploration for parasitoids, predators, and pathogens the insect’s country of origin. Research on biology, potential efficacy, rearing, host specificity, and completion of an Environmental Assessment and public comment period are required. Depending on the risk assessment outcome for each natural enemy, USDA APHIS may or may not issue permits for release of exotic natural enemies in the U.S. A survey of extant ALB natural enemies in the U.S. may reveal important native natural enemies or those introduced and established with ALB from China or Korea.

Research Results

In the U.S., insect pathogenic fungi were the most diverse and prevalent natural enemy of ALB including Beauveria bassiana,Metarhizium anisopliae, Verticillium lecanii, and Paecilomyces farinosus. In addition, an unidentified species of Entomophthorales was isolated from an adult cadaver dissected from a tree in Chicago and in eggs collected in New York.

In China, the most widespread natural enemy isolated from ALB was an unidentified species of microsporidium. Microsporidia are a group of specialized obligate, intracellular pathogens of insects and other eukaryotic organisms. Once lumped together as protozoans with other single-celled eukaryotes, genetic analyses suggest they are related to fungi. Although microsporidiosis may cause mortality, many infected individuals sustain chronic, debilitating infection. Those with chronic disease, release spores into the environment and infect others in their cohort. Many insect microsporidia are also transmitted from mother to progeny in their eggs. ALB populations in China had an average infection prevalence of ca. 5%, ranging from <1% during the spring to 11% during the fall. This seasonal increase likely resulted from the high density ALB populations at our field sites. By the end of the season, larval galleries usually overlapped in the crowded tree trunks, increasing the probability of healthy larvae ingesting microsporidian spores released by diseased and moribund larvae.

A second insect pathogen, Beauveria bassiana, was isolated from infected larvae at two field sites during the spring. In subsequent work, we studied the pathogenicity of various entomopathogenic fungi to ALB adults (see pub Dubois et al. 2007) and research on the use of fungal bands for management of ALB continues (see pubs Hajek et al. 2007; Hajek & Bauer 2009).
Only one parasitoid was found attacking ALB: Dastarcus longulus (Coleoptera: Colydiidae), a larval ectoparasitoid, was found attacking ALB larvae and pupae during the spring at one site, with ca. 5% parasitized.
Possible predators isolated in ALB galleries included ants (Formicidae) and a minute pirate bug (Anthocoridae).In both the U.S. and China, ALB larvae supported a surprisingly high number of phoretic deutonymphs of astigmatic mites. At one site in China, we found phoretic deutonymphs mesostimatic mites attached to adult ALB. Despite their abundance on some larvae, phoretic mites apparently do not adversely affect their hosts.

Mites infesting head of ALB larva (photo credit: H. Liu)

Additionally, some ALB larval cadavers recovered from galleries contained nematodes, however, we were unable to confirm if these were parasitic or saprophytic.